Collet chucks provide a simple, inexpensive method for gripping and transmitting torque to a straight, cylindrical shank of a tool. The collet typically has a tapered outer surface and a straight axial bore for receiving the cylindrical shank of the tool. The collet includes a plurality of axial slots with alternating front and rear ties which are equally spaced around the collet. A lock nut is used to apply an axial load to the collet to force the collet into a tapered bore in a holder. The tapered wall of the bore collapses the collet causing it to grip the shank of the tool.
Collet chucks of the type described rely on the gripping force generated by the collet to resist axial and radial movement of the tool. In certain machine operations where high torque loads are generated, the gripping force generated by the collet may not always be sufficient to prevent radial slipping. For this reason, some types of tools, such as thread-forming taps, have a square drive on the end of the shank. Collets have been specifically designed to grip on the square drive of the shank to prevent radial slipping of the tap and to provide positive torque transmission.
One method for gripping the square drive of the tap is to broach a square hole in the collet for receiving the square drive of the tap. Another type of collet receives the four corners of the square drive in the axial slots of the collet. This type of collet obliquely engages the four corners of the square drive to transmit torque to the tool. Each of these methods allow some radial movement of the tap which is undesirable and do not always align the tool accurately. Also, collets using these methods require additional machining operations and extra parts which increases the cost of producing the collet.